Existing and emerging consumer applications have created an increasing need for real-time three-dimensional imagers. These imaging devices, also commonly known as light detection and ranging (LiDAR) sensors, enable the remote measurement of distance (and often intensity) of each point on a target scene—so-called target scene depth—by illuminating the target scene with an optical beam and analyzing the reflected optical signal. A commonly used technique to determine the distance to each point on the target scene involves sending an optical beam towards the target scene, followed by the measurement of the round-trip time, i.e. time-of-flight (ToF), taken by the optical beam as it travels from the source to target scene and back to a detector adjacent to the source.
A suitable detector for ToF-based LiDAR is provided by a single-photon avalanche diode (SPAD) array. SPADs, also known as Geiger-mode avalanche photodiodes (GAPDs), are detectors capable of capturing individual photons with very high time-of-arrival resolution, of the order of a few tens of picoseconds. They may be fabricated in dedicated semiconductor processes or in standard CMOS technologies. Arrays of SPAD sensors, fabricated on a single chip, have been used experimentally in 3D imaging cameras. Charbon et al. provide a useful review of SPAD technologies in “SPAD-Based Sensors,” published in TOF Range-Imaging Cameras (Springer-Verlag, 2013), which is incorporated herein by reference.
In a SPAD, a p-n junction is reverse-biased at a level well above the breakdown voltage of the junction. At this bias, the electric field is so high that a single charge carrier injected into the depletion layer, due to an incident photon, can trigger a self-sustaining avalanche. The leading edge of the avalanche current pulse marks the arrival time of the detected photon. The current continues until the avalanche is quenched by lowering the bias voltage down to or below the breakdown voltage. This latter function is performed by a quenching circuit, which may simply comprise a high-resistance ballast load in series with the SPAD, or may alternatively comprise active circuit elements.